A tail of sodium atoms trails the planet Mercury in these observations from the Messenger spacecraft. The solar wind knocks the atoms off of Mercury's surface, and solar radiation pushes them away from the planet to form a long, thin tail. [NASA/JHU/APL]

The planet Mercury is about as dull as a box of rocks. In fact, it basically is a box of rocks. It has a large core of iron and nickel, surrounded by lighter-weight rocks, many of which have a volcanic origin.

Yet Mercury does have one dynamic feature: a tail that can stretch a million and a half miles behind it. It's made mainly of sodium, one of the elements in table salt. It comes from the surface of Mercury, but it's stirred up by the space environment.

The process is known as space weathering. It happens when the surface is eroded by outside forces: impacts by tiny space rocks, for example, or the steady spray of the solar wind.

In Mercury's case, the solar wind seems to play the biggest role. This stream of particles from the Sun is funneled toward the surface by Mercury's magnetic field. As the particles hit the surface, they knock off atoms of sodium and other elements. Radiation pressure from the Sun then blows many of these atoms away from Mercury -- perhaps a few ounces of material per second.

The tail glows in the sunlight. But the tail is so thin that you need a telescope and sensitive detectors to see it.

You won't need any help to see Mercury itself the next few evenings because it's quite near Venus, the brilliant "evening star." They pop into view low in the west not long after sunset. Tonight, Mercury is just to the lower right of Venus. It looks like a fairly bright star.

More about Venus and Mercury tomorrow.

Script by Damond Benningfield, Copyright 2010

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